Magnetotactic bacteria are ubiquitous and can be found in both freshwater and marine environments. Due to intracellular chains of magnetic single domain particles, they behave like swimming compass needles. In external magnetic fields like the Earth's magnetic field, a torque is acting on the chain. This will cause the bacterium to be rotated and aligned with the external field. The swimming direction of magnetotactic bacteria can be controlled with external magnetic fields, which makes it convenient to study them under a light microscope. Usually, a special set of coils arranged around a light microscope is used to control the swimming magnetotactic bacteria. Here, we present a simple mechanical system with a permanent magnet, which produces a rotating magnetic field of nearly constant amplitude in the focal plane of a light microscope. The device is placed beside the light microscope and easily adaptable to almost any microscope and thus convenient for field experiments. To describe the trajectories qualitatively, a theoretical model of the trajectories is presented. This device can be used to control the swimming direction of magnetotactic bacteria and also for studying their magnetic and hydrodynamic properties.
In reflection laser scanning microscopes, detection of odd aberrations is challenging because aberration cancellation can occur after the second passage of the light beam through the system. A method is proposed that uses a sample containing high spatial frequencies, such as an edge scan, to detect and measure the presence of odd aberrations. The new approach is demonstrated by scanning the focal spot over an edge in a confocal reflection microscope when coma is present in the imaging system (a common odd aberration). It is shown that the edge response displays characteristic distortions which are typical of coma. Detection of amplitude, sign and orientation of the coma aberration is made possible by comparison of the measured edge responses with theoretical curves.
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